Zero Mischa

Zero, short for "patient zero", is a sentient parasite colony. It seems determined to act like a normal sentient creature, which is why it refuses to change its name as it's the one that was assigned to it at "birth" (or, at least, when it established itself enough to become intelligent), taking its parent's (host) surname and insists on being considered as a single entity and even assigned itself a gender (male). 

He is incapable of reproduction - or, more specifically, incapable of siring new colonies, though he can and does produce individual parasites. How it even managed to take over its host is a mystery to itself and every medical team that ever had the chance (or authorisation) to study him; the most plausible explanation is that he is a mutant, but his biology is extremely complex and there isn't any other colony like him to compare him to. His preferred host is unknown, having been contracted at the heart of a space hulk and is theorised to be either extinct or some kind of space creature. He does feel lonely and that's perhaps what drives him to try to act like other sentient creatures around him.

Zero is very harsh on his body and requires a combination of surgery and regular feedings to maintain himself in peak condition, although his mucosae tends to bleed on and off regardless of his state.